Treatment of continuous lengths of metal by electrical resistive heating

ABSTRACT

An apparatus for heat-treatment of continuous lengths of metal, particularly continuous bands, strips, sheets or wires of ferrous metal, to effect the annealing or tempering thereof, and/or the reduction of the oxides thereon, preparatory to the passage thereof through a liquid treating bath, for example, a molten metal coating bath, such as aluminum, zinc, tin or the like, if an effective protective corrosion-resistant coating on the ferrous metal is sought to be attained. The continuous length of metal is heated by the direct application of electrical energy thereto as the same travels between conveyor or guide rolls to which the electrical potentials are applied to generate heat, by the Joule effect, in the transient lengths between the rolls, while the metal travels in otherwise unheated chambers, which may contain a suitable protective gaseous atmosphere. The spacing between the conveying rolls is diminish-ed, corresponding to the increasing thermal content of the travelling length of metal beyond the inlet and towards the outlet, and the eventual treating bath, to equalize the electrical heat energy, represented by the product I2R, in the successive lengths between the rolls, to compensate for the higher resistivity of the hotter lengths of metal in the advanced path thereof, so that the current loadings of the multiple stages of the power supply are substantially equalized.

United States Patent 1191 J anatka et al.

[ Feb. 19, 1974 1 TREATMENT OF CONTINUOUS LENGTHS OF METAL BY ELECTRICAL RESISTIVE HEATING [75] Inventors: Vladimir Janatka, Woodbury;

James J. Dolan, Southport, both of Conn.

[73] Assignee: Thomas Dolan, Fairfield, Conn.

a part interest by said James J. Dolan 22 Filed: Mar. 19, 1973 21 Appl. No.: 342,818

2,162,692 6/1939 Baily 266/3 R 2,176,583 10/1939 Cook 266/3 R 2,502,770 4/1950 Watson..... 118/620 2,894,115 7/1959 Alt 219/155 3,353,806 ll/l967 Lichte 266/3 R 3,518,109 6/1970 Halley 118/620 X 3,565,683 2/1971 Morelock 118/620 X 3,592,163 7/1971 Bauer 118/620 3,721,520 3/1973 Bloom 432/143 Primary ExaminerVolodymyr Y. Mayewsky Attorney, Agent, or Firm-Samuel Lebowitz [57] ABSTRACT An apparatus for heat-treatment of continuous lengths of metal, particularly continuous bands, strips, sheets or wires of ferrous metal, to effect the annealing or tempering thereof, and/or the reduction of the oxides thereon, preparatory to the passage thereof through a liquid treating bath, for example, a molten metal coating bath, such as aluminum, zinc, tin or the like, if an effective protective corrosion-resistant coating on the ferrous metal is sought to be attained. The continuous length of metal is heated by the direct application of electrical energy thereto as the same travels between conveyor or guide rolls to which the electrical potentials are applied to generate heat, by the Joule effect, in the transient lengths between the rolls, while the metal travels in otherwise unheated chambers, which may contain a suitable protective gaseous atmosphere.

The spacing between the conveying rolls is diminished, corresponding to the increasing thermal content of the travelling length of metal beyond the inlet and towards the outlet, and the eventual treating bath, to equalize the electrical heat energy, represented by the product PR, in the successive lengths between the rolls, to compensate for the higher resistivity of the hotter lengths of metal in the advanced path thereof, so that the current loadings of the multiple stages of the power supply are substantially equalized.

6 Claims, 5 Drawing Figures PATENTEDFEBI 9 m4 3'. 792 .6 84 same 'or PAIENTED FEB 1 91974 I 3.792.684 SHEET 30F a QEQxUOnS QE PAIENTEU 3792684 I saw u [If 4 TREATMENT CONTINUOUS LENGTHS OF METAL BY ELECTRICAL RESISTIVE HEATING Theinvention is concerned with the heat treatment of continuous lengths of metal for the purpose of controlling the physical properties thereof, either independently, or in conjunction with additional treatments of the lengths of metal, for example, the annealing or tempering thereof and/or the reduction of oxides that may be formed thereon, with or without the application of coatings thereto.

It is the object of the present invention to provide a highly compact and economical installation for the heat treatment of continuous lengths of metal for the purpose of imparting accurately controlled degrees of heat thereto for the purpose of modifying the physical properties of the metal, which installation may be complemented by additional apparatus for tempering, annealin g or chemically treating the metal for further processing such as quenching, pickling or coating procedures.

It is also the object of the invention to provide a method of heat treating continuous lengths of metal, which method is capable of accurate and sensitive control of the thermal characteristics of the continuous lengths of metal to attain the desired results.

It is a further object of the invention to provide an apparatusfor the heat treatment of continuous lengths of metal which occupies a a minimum amount of floor area, which may be built up of low cost modular structural units, and which may be maintained in service for maximum periods of time without costly shut-downs when interruptions or breakdowns occur.

The invention contemplates the heat treatment of continuous lengths of metal by means of electrical enegy which is applied to the travelling metal directly by electric potentials imposed upon conveyor or guide rolls for the metal in multiple stages, the lengths of which are varied to equalize the energy consumed in the respective stages, which is expended in heat by the Joule effect in the metal, as it travels between the several guide rolls or pulleys.

It is a further object of the invention to provide an installation which is of particular utility in the heat treatment of continuous lengths of ferrous metal in the form of sheets, bands, strips and wires which are heat treated preparatory to the coating thereof with another metal such as aluminum, zinc, tin or the like, which procedure requires the effective cleaning of the surface of the metal to remove the oxides therefrom. This requires the passage of the continuous length of metal through chambers containing a protective gaseous atmosphere which is non-oxidizing or reducing in chemical behavior, which treats the travelling length of metal in the course of its advance towards a molten metal coating bath. The protective gas is introduced into the chambers for travel in countercurrent relation to the direction of the travelling length of metal, to increase the efficiency of the system as the metal is first heated accurately to the desired temperature, followed by the cooling thereof and the hot dipping of the metal for the application of the coating thereto, in the course of its passagefrom the inlet to the outlet of the apparatus.

The invention contemplates the economical heat treatment of continuous lengths of ferrous metal preparatory to the passage thereof through coating baths of molten metal which are treated for the purpose of clearing the metal of objectionable oxide layer, with or without the annealing of the metal. Alternatively, the heat treatment of the continuous lengths of ferrous metal may be executed perparatory to the passage of the crtically heated metal through quenching baths, if tempering characteristics are sought to be imparted to the metal, or other liquid baths such as pickling solu tions and the like.

The present invention presents a marked improvement over the heat treatment of continuous lengths of ferrous metal for annealing the metal and reducing the oxides thereon preparatory to the hot dip galvanizing thereof, as disclosed in the US. Pat. to Sendzimir, Nos. 2,136,957 and 2,224,410. The heat-insulated tunnel furnaces into which a reducing agent such as hydrogen is introduced, as are disclosed in these patented installations, are costly to build and to maintain, and are inefficient in operation. Such furnaces are heated by gas, oil or electrically to about 1,900F. in order to impart to the metal and surrounding protective gas a temperature sufficiently high to effect the annealing of the metal and the reduction of the oxide, which is generally in the range of 1,800F. Higher temperatures than that attained by the metal and coatings must be generated in the furnace, in order to impart the desired heat to the travelling band, so that a high heat inertia is always present. Any breakages in the band require stoppages of long duration for repair, to permit the cooling of the furnace. The excessive heating of the furnace parts, to impart the desired heat to the products being treated, contributes to the deterioration of the installation and the attendant costs of the operation.

The above-mentioned disadvantages are overcome by the utilization of guide of conveying rolls for the travelling lengths of metal which are bridged by ducts, to sub-divide the travelling lengths of metal into a plurality of stages, to which electrical power is applied directly, so that all of the energy is expended in the direct heating of the metal as it moves between the guide rolls onto which are impressed the electrical potentials. The variation of the heat content of the travelling metal as it proceeds through the apparatus, is utilized to control the electrical loading in the severalstages, so that maximum efficiency of electrical power is attained. The chambers through which the travelling metal passes successively are unheated, other than by heat radiation from the metal as it travels therethrough, which factor may be taken into account in calculating the length of the stages between successive guide rolls as the metal travels from the inlet to the outlet of the apparatus.

In view of the fact that the chambers or ducts extending between the successive guide rolls require no extraneous heating elements, such as those normally required for energization by gas, oil or electricity, the cross-section thereof may be reduced to accommodate only the traveling band of material. This results in an economical utilization of the gases which generate the protective atmosphere surrounding the bands of metal which may be either chemically neutral or reducing in behavior.

The initial efficient heating of the travelling band makes possible utilization of minirrfiim heat insulation adjacent to the inlet end of the apparatus, that is, in the heating-up zone, and a variable amount of insulation in the subsequent zones, depending upon the character of the treatment desired for the travelling band of metal following its attainment of the desired heat.

' travelling band of metal renders possible a quick variation of the heat treating programs of the metal to attain any desired objectives. For exmaple, the metal may be heat treated to such a degree that an annealing thereof occurs, which heating may be reduced by controlling the voltages applied to the guide rollers to eliminate any annealing action and to confine the heat treatment to the reduction of the oxide coating alone.

The arrangement of the heat treating apparatus in accordance with the invention simplifies starting and stopping of the heat treatment of the lengths of metal, without the danger of over-annealing or burning of the portion of the strip which remains in the apparatus during a shutdown as is the case with a tunnel furnace. The re-threading of the continuous travelling length of metal in the case of the breakage of the latter, is also greatly simplified because the threading operation is not hampered by the inaccessibility to the furnace which is heated to an elevated temperature, since the arrangement in accordance with the invention cools down in a matter of minutes rather than hours as is the case with a space-heated tunnel furnace,

Other objects and purposes will appear from the detailed description of the invention following hereinafter, taken in conjunction with the accompanying drawings, wherein FIG. 1 is a schematic cross-sectional view of the apparatus in accordance with the invention, indicating the line of travel of a continuous length of metal as it passes through the heat treating stages, cooling stages, and ultimately a coating bath;

FIG. 2 is a transverse sectional view through a guide roll or pulley over which the length of metal travels;

FIG. 3 is a schematic diagram indicating the manner of electrical energization of the guide rolls adjacent to the inlet of the apparatus;

FIG. 4 is a schematic diagram illustrating a variant mode of energizing the guide rollers adjacent to the inlet of the apparatus; and

FIG. 5 is a schematic diagram of an installation similar to that shown in FIG. 1, including the three-phase alternating current power supply for the guide rollers adjacent to the inlet of the apparatus and the arrangement of the gas generating components for producing the protective atmosphere for the continuous length of metal travelling through the apparatus as it passes from the inlet through a galvanizing coating bath.

In FIG. I is shown schematically the travelling length of metal M which may be in the form of sheets, bands and strips of any width, or wires of different crosssections and dimensions, which are adapted to be heattreated in the apparatus in accordance with the invention, and which, in the illustrated embodiment, are adapted to pass through a coating bath, followed by their withdrawal from the apparatus for accumulation on reels or other desired disposition.

The continuous length of metal M passes under the roller R1 and upwardly therefrom around pulley or guide roll 23 wherefrom it is guided through a duct 2 towards deflecting pulley 19 within housing 13 adjacent to the floor of the installation. Housings 10, 1 1 and 12 are mounted on the framework at raised levels of the apparatus, and housings l4 and 15 are mounted on the framework beyond housing 13 at lower levels of the apparatus, so that the length of metal M may pass upwardly through duct 3 extending between housings l3 and 10 for guidance around pulley 16 in the latter, wherefrom the same is directed downwardly through duct 4 extending between housings 10 and 14. The guide pulley 20 in the latter deflects the length of metal M upwardly for passage through the duct 5 extending between housings l4 and l1, and the guide roller 17 in the latter deflects the length of metal M downwardly through duct 6 extending between housings 11 and 15. The length of metal is then guided around guide roller 21 for passage through the duct 7 which extend between housings 15 and 12 and is guided from the latter in a downward direction by deflecting pulley 18, for passage through housing 45 in which is disposed the guide roller 22 which deflects the band of metal M past the guide roller 22 into duct 9. The lower end of duct 9 is submerged in the treating bath 39, wherein is disposed the guide roller 41 which directs the band in an upward direction for passage around a portion of the guide roller R2 wherefrom the same is led in the direction of the arrow for disposition on a reel or other point of collection. 7

The guide pulley 23 at the inlet of the apparatus is not enclosed within any housing, and the band M travelling downwardly therefrom is introduced into the admission port 24 at the upper end of the duct 2 which defines the beginning of the chamber containing the protective atmosphere, which may be chemically neutral or non-oxidizing. This gaseous atmosphere is fed into the ductwork through opening 25, shown in greater detail in FIG. 5, adjacent to the outlet end of the ductwork. This gaseous atmosphere may consist of nitrogen with an addition of hydrogen gas which imparts to the gas an oxide-reducing capability.

The several pulleys or guide rollers described above, which are enclosed within the several housings, are constructed in the manner illustrated in FIG. 2, and are designed to be electrically conductive for energizing the metal band travelling therearound while being electrically insulated from the housings surrounding them. For this purpose, the metal roll 16, which is illustrative of the other guide rolls in the installation, is mounted on shaft 28 which is journalled in bearings 29 and 30 of heat-resisting material such as carbon. These bearings are in turn insulated from the housing 10 by means of asbestos rings 26 and 27 on the opposite ends of the housing. A collector ring 31, of conductive metal, for example copper, is mounted on one end of shaft 28 and power is fed thereto by means of brushes 32 in contact therewith. In order to allow freedom of expansion of the guide pulley 16, the pulleys are mounted on shafts 28 by means of flexible diaphragms 38. The opposite end of the shaft 28, within housing 10, is sealed by means of a cover 37 in order to maintain the protective gas within the ductwork.

The heating of the metallic bands or wires by the travel thereof between guide rollers on which are imposed electric potentials, so that the bands become part of the circuit resistance path, and are heated by the Joule effect, is a practice that has been known in the industrial arts for many years, and is disclosed in the US. Pats. to Cook, Nos. 2,176,583 and 2,320,412, as well as in the US. Pat. to Bower, No. 3,592,163, which are illustrative of many others. However, no one heretofore conceived the advantages accruing from the utilization of the increased resistivity of the travelling length of metal as it passes from the cooler toits hotter stages, and to employ this phenomenon for equalizing the power consumption in the several stages of the power supply by selectively adjusting the lengths of metal extending between the respective guide rollers. Thus, as shown in FIG. 1 and FIG. 3, the three-phase power supply 34, which may be the secondary windings of a transformer, is balanced in its three-phases 34a, 34b and 34c by varying the lengths of the passes of metal extending between the guide rollers so that these decrease in length from the inlet towards the outlet of the apparatus. Thus, the first pass of the traveling band M2, which extends from pulley 23 to pulley 19, is relatively cool and'is longer than the second pass of metal M3 extending from pulley 19 to pulley 16, which in.

turn is longer than the third pass of metal M4, so that the successive increase in resistivity in the three successive passes results correspondingly in the same currents being drawn by the voltages impressed across each length between rollers 23 and 19, between 19 and 16, and between 16 and 20, so that the loadings on the three phases of the source of power are substantially the same, despite the fact that the band of metal grows hotter as the band moves along in its path from the inlet to the outlet of the installation.

An alternate mode of energizingthetravelling band of metal between the stages extending between pulleys 23 and 19 and pulleys l9 and 16, is illustrated in FIG. 4. In this case, the alternating current power supply P is fed to a pair of rectifier bridges B1 and B2. The output of the bridge B1 is connected to the-collector rings on guide pulleys 19 and 23, whereas the output of the second bridge rectifier B2 is connected between the pulleys l9 and 16. The length of metal M2 between pulleys 23 and 19 is longer than the length of metal M3 between pulleys l9 and 16. In view of the fact that the metal in the length M2 is cooler than that in M1, with a consequent lower resistivity, the current factor in the former is higher, so that the proper adjustment of the relative lengths of travelling metal M2 and M3, the Joule effect, equivalent to PR is substantially constant, which results in a more efficient utilization of power.

The utilization of the variable electrical resistivity of the travelling band of metal corresponding to its temperature, gives rise to a self-regulating feature of the system. With a proper blance'between the energy input and the speed of the travelling metal, the latter does not become overheated because any temperature increase beyond the intended level automatically results in a higher resistivity which in turn results in a redcution of current and energy consumption to impart a stability to the system.

As shown in FIG. 5, the three-phase alternating current power is imposed across pulleys 23, 19 and 16 and, as explained .above, results in a gradual increase in the temperature of the metal band M as it passes through the heatingzone. The pulley 23 is not enclosed within a housing because at this point the metal band or strip is at a low temperature and experiences no oxidation.

Upon entry of the strip into the upper end of duct 2 through slot 24, it is surrounded by the protective gaseous atmosphere which may be supplied by a nitrogen generator 44 or any other suitable exothermic gas generator. Hydrogen is added to the inert gas in a relatively small percentage, which may be about 10 percent. This source of hydrogen may be an ammonia dissociator which is mixed together with the nitrogen in the mixing chamber 36, whence both gases in the desired proportions, as controlled by the valves V shown in FIG. 5, are fed to the ductwork 9 adjacent to the outlet by means of coupled inlet 25. The gas mixture is maintained at a slight over-pressure in the ducts 2 to 9 and the housings connecting therewith, in order to avoid any leaks of atmospheric oxygen into the system from the surrounding area. As described above, the housings are hermetrically sealed by covers 37 as well as flexible seal 38 and by the liquid seal in bath 39. The only opening in the ductwork is at the slot 24 at the upper end of duct 2 through which the gas is exhausted and burnt off. When the system is employed for the heat-treatmentof ferrous bands of metal, preparatory to the application of a protective metal coating thereto, the exhaustion of gas is necessary in order to expel from the chamber the water vapor formed by the chemical reaction between the hydrogen and iron oxide so that a low humidity level inside the chamber is maintained for the efficient reduction of said oxides. The burn-off or combustion of the gas is beneficial because it effectively preheats the metal band M before it enters through the slot 24.

The temperature of the strip is controlled by radiation and/or optical pyrometers, provided in windows 40 strategicaly located in the enclosures 10 to 13 and 14. During normal operation the temperature of the strip is maintained at about l,850F. and this temperature is measured in the enclosure 14, just before the strip contacts the pulley 20. At this temperature not only the reduction of oxides is achieved, but also complete recrystallization and therefore annealing of the strip takes place. In some instances however, annealing of the strip is not desirable. If the strip is to be used, for example, for fabricating corrugated roofing sheets, the material should retain the rigidity of work-hardened steel. In this case, the temperature of the strip passing through the chamber can be reduced to about l,l00F. simply by decreasing the current fed into the heating circuit or by increasing the linear velocity of the strip passing through said circuit, so that each portion of said strip is subjected to a shorter heat cycle.

After the strip passes over the last electrified pulley 20, it enters into the cooling zone. The ducts S, 6, 7, 8 and 9 are not heat-insulated to allow maximum heat dissipation. The pulleys l7, 18, 21 and 22 are not electrified. The strip passes over these pulleys and is finally deflected by pulley22 and enters into inclined duct 9, the extremity of which is submerged within the molten metal coating bath contained in the pot 39. The strip comes into contact with said coating metal without being exposed to the atmosphere. Due to the absence of oxides, which have been reduced during the passage of the strip through the chamber, the coating metal adheres tightly to the surface of the strip, forming an alloy with the base metal. From the pot 39, the strip passes under or around the deflector roll 41 and passes from the bath into the ambient atmosphere in the form of a coated band.

Because the'reduction chamber is filled partly with hydrogen, precaution must be taken during the normal operation and especially during the start-up and shutdown of the unit. Start-up must be preceded by thorough purging of the chamber with nitrogen, until a sample of gas taken at the entry slot 24 indicates that there is no oxygen present in the exhausted gas. Glow elements 42, inserted in the pulley enclosure and near the entry slot 24, must be activated prior to the inclusion of hydrogen into the purging gas. The function of these glow elements is to ignite any residual oxygen which may remain after purging inside the chamber. During the operation an over-pressure of about 2 or 3 inches of watercolumn should be maintained within the chamber in order to eliminate any possibility of oxygen leaking into the chamber. The glow elements should remain active during the cycle of operation and in case any pressure drop occurs within said chamber, the pressure must be quickly restored by opening the nitrogen emergency by-pa ss valve 43 (FIG. 5). Shut-down of the unit must also be preceded by thorough purging of the chamber by nitrogen. The purge should be discontinued only when all hydrogen from the chamber is exhausted.

While the illustrated embodiment discloses specially a simplified installation for continuous annealing or hot-dip coating of ferrous bands, strips or wires, the heat treatment of the bands in successive heat stages, which are energized electrically by the application of electrical potentials to the guide rolls over which the lengths of metal pass, with the consequent advantages of the efficient utilization of electric power, may be applied to the heat treatment of lengths of metal without any subsequent treatments. Furthermore, the heat treatment procedures in accordance with the invention may be applied to continuous lengths of metal in conjunction with the tempering of ferrous metals or modifying the physical characteristics of the metals by subjecting such metals to a treating bath which may assume the form of quenching baths or pickling baths.

We claim:

1. An apparatus for heat-treating a continuous length of a flexible metal element travelling in an unheated space containing a protective non-oxidizing gas, comprising a. a plurality of metal rollers for conveying the continuous length of the metal element along a plurality of zig-zag paths from an inlet to an outlet,

b. electric power means for applying electrical energy across some of said rollers adjacent to the inlet to pass electric current therethrough'and to heat directly portion of the length of the metal element extending transiently between adjacent rollers, by the Joule effect,

c. sealed chambers enclosing said rollers with ducts therebetween for confining said gas, the length of said ducts and the spacing between said electrically energized rollers being so adjusted as to progressively reduce the length of said paths therebetween in correspondence to the remoteness from said inlet, to equalize the electric power consumption across adjacent rollers despite the increase in resistivity of the traveling length of the metal element as it increases in heat content beyond the inlet end,

d. insulating mounting means for said rollers, chambers and ducts, and

e. means for supplying said protective gas to said sealed chambers and ducts.

2. Apparatus as set forth in claim 1, wherein said electric power means comprises a source of threephase alternating current with connections from each phase across each of the first three rollers adjacent to the inlet of the apparatus.

3. An apparatus as set forth in claim 1, wherein said electric power means comprises a source of alternating current and a pair of rectifying systems connected thereto for energizing the first two paths of said travelling length of the metal element.

4. An apparatus as set forth in claim 1, wherein said last-mentioned means is connected to the ducts adjacent to the outlet for travel through said ducts and sealed chambers in a direction countercurrent to the travel of the length of the metal element therethrough.

5. An apparatus as set forth in claim 4, including a liquid treating bath beyond the connection of said protective gas supply with said ducts.

6. An apparatus as set forth in claim 1, including a molten metal coating bath at said outlet for the passage of the heat-treated length of the metal element therethrough, and an inlet for the protective gas supplied to said sealed chambers and ducts in advance of said coating bath for travel through said ducts in a direction opposite to that of said length of the metal element. 

1. An apparatus for heat-treating a continuous length of a flexible metal element travelling in an unheated space containing a protective non-oxidizing gas, comprising a. a plurality of metal rollers for conveying the continuous length of the metal element along a plurality of zig-zag paths from an inlet to an outlet, b. electric power means for applying electrical energy across some of said rollers adjacent to the inlet to pass electric current therethrough and to heat directly portions of the length of the metal element extending transiently between adjacent rollers, by the Joule effect, c. sealed chambers enclosing said rollers with ducts therebetween for confining said gas, the length of said ducts and the spacing between said electrically energized rollers being so adjusted as to progressively reduce the length of said paths therebetween in correspondence to the remoteness from said inlet, to equalize the electric power consumption across adjacent rollers despite the increase in resistivity of the travelling length of the metal element as it increases in heat content beyond the inlet end, d. insulating mounting means for said rollers, chambers and ducts, and e. means for supplying said protective gas to said sealed chambers and ducts.
 2. An apparatus as set forth in claim 1, wherein said electric power means comprises a source of three-phase alternating current with connections from each phase across each of the first three rollers adjacent to the inlet of the apparatus.
 3. An apparatus as set forth in claim 1, wherein said electric power means comprises a source of alternating current and a pair of rectifying systems connected thereto for energizing the first two paths of said travelling length of the metal element.
 4. An apparatus as set forth in claim 1, wherein said last-mentioned means is connected to the ducts adjacent to the outlet for travel through said ducts and sealed chambers in a direction countercurrent to the travel of the length of the metal element therethrough.
 5. An apparatus as set forth in claim 4, including a liquid treating bath beyond the connection of said protective gas supply with said ducts.
 6. An apparatus as set forth in claim 1, including a molten metal coating bath at said outlet for the passage of the heat-treated length of the metal element therethrough, and an inlet for the protective gas supplied to said sealed chambers and ducts in advance of said coating bath for travel through said ducts in a direction opposite to that of said length of the metal element. 